Organic EL elements utilizing electroluminescence of organic materials have drawn attention as display elements used for thin display devices. Organic EL elements emit light when holes injected from the anode and electrons injected from the cathode recombine within a light-emitting layer disposed between these electrodes. Such self-luminous organic EL elements have advantages such as high-luminance light emission, a high response speed, a wide viewing angle, a thin profile, and a light weight, and are therefore expected to be applied to various fields such as display panels and illumination lamps.
The light-emitting layer of an organic EL element is usually a mixed layer, obtained by co-deposition, of a luminescent dopant material mainly providing luminescence and a luminescent host material mainly transporting holes and electrons. Studies have been made to enhance the functionality of such organic EL elements. For example, the following organic EL elements are known.
Patent Literature 1 discloses an organic EL element including a common light-emitting layer as described above and a layer whose luminescent dopant material concentration is 100 wt %.
Patent Literature 2 discloses an organic EL element including a common light-emitting layer and a layer that mainly contains a luminescent host material and does not contain a luminescent dopant material.
Patent Literature 3 discloses an organic EL element including, in place of a common light-emitting layer, an island-shaped thin film that is made of a fluorescent organic compound serving as the center of recombination to emit light and that has a smaller average thickness than a monomolecular film of a fluorescent organic compound.
Patent Literatures 4 and 5 each disclose an organic EL element including a common light-emitting layer and a metal-doped layer, obtained by doping with a metal or a metal salt, between the cathode and an organic compound layer that is disposed between the anode and the cathode.
Patent Literature 6 discloses an organic EL element including a common light-emitting layer made of a material with predetermined properties and a barrier layer with predetermined properties. Patent Literatures 7 and 8 each disclose an organic EL element including a laminate of common light-emitting layers made of a material whose properties are utilized as predetermined properties.
Non Patent Literatures 1 and 2 each disclose an organic EL element including a common light-emitting layer and an efficiency-enhancement layer.